Conventional aircraft accelerating during ground run at take-off from zero airspeed to lift-off airspeed, encounter very low Reynolds numbers at the start of acceleration Similarly, VTOL VSTOL and STOVL aircraft transitioning from hovering vectored flight to aerodynamic flight also essentially accelerate from zero forward airspeed to cruising airspeed, for example, and similarly encounter very low Reynolds numbers at the start of acceleration.
At such very low Reynolds numbers, unfavorable flow phenomena can be encountered, referred to herein as the unfavorable flow phenomena domain. Such phenomena include separated flow over the wings at low Reynolds numbers, including laminar leading edge stall and the burst of laminar separation bubble, which can sometimes distort the aerodynamics characteristics of the air vehicle, leading to problems of flight safety and radical degradation of air vehicle performance including poor lift and high drag. The range of Reynolds numbers associated with the unfavorable phenomena domain is typically about 0.1*106 to about 0.5*106.
For large conventional aircraft, having wings with aerofoil sections of relatively large chords and operating at relatively high cruising airspeeds (with correspondingly high Reynolds numbers), the stage of experiencing such low Reynolds numbers during aircraft acceleration is very short, and occurs while the aircraft is still not airborne. Such large conventional aircraft thus achieve very quickly relatively high Reynolds numbers that are beyond the low Reynolds numbers range corresponding to the unfavorable flow phenomena domain, and thus in a practical sense such phenomena do not affect the operation of such aircraft.
On the other hand, the situation is rather different for small aircraft, such as for example Tactical UAV (for example having a weight of between 10 kg and 300 kg). In such small aircraft, the relatively low final airspeed after the stage of acceleration, combined with small wing chords, can correspond to relative small Reynolds numbers within the low Reynolds number range corresponding to the unfavorable flow phenomena domain, which conventionally leads to the aforesaid problems.